1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method for use in a lithographic apparatus.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In this case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC. This is done using a projection system that is between the reticle and the substrate and is configured to image an irradiated portion of the reticle onto a target portion of a substrate. The projection system includes components to direct, shape and/or control a beam of radiation emitted from a suitable source. The pattern can be imaged onto the target portion (e.g. including part of one or several dies) on a substrate, for example a silicon wafer, that has a layer of radiation-sensitive material, such as resist. In general, a single substrate contains a network of adjacent target portions that are successively exposed.
Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and scanners, in which each target portion is irradiated by scanning the pattern through the beam of radiation in a given direction, usually referred to as the “scanning” direction, while synchronously scanning the substrate parallel or anti-parallel to this direction. Examples of known systems are described in U.S. Pat. Nos. 6,452,661 and 6,570,168 and U.S. Patent Application Publication 2004/0036037 A1.
A criteria considered in lithography is the size of features of the pattern applied to the substrate. It is desirable to produce apparatus capable of resolving features as small and close together as possible. A number of parameters affect the available resolution of features. One of the most important of these is the wavelength of the radiation used to expose the pattern. Using radiation with an EUV (Extreme Ultra Violet) wavelength between 5 and 20 nm, and typically 13.5 nm, it is anticipated that it will be possible to manufacture of feature sizes down to 32 nm.
Various EUV sources are known. For example, some plasma-based radiation sources emit radiation in this wavelength range. These sources are generally volume radiators, i.e. they simultaneously emit radiation in all directions. Plasma sources include a discharge medium that can be stimulated to form a plasma and emit radiation either by using suitable laser radiation or by using an electrical discharge. These sources come in many different forms and use many different materials for the discharge medium. Examples of suitable discharge materials include xenon, lithium, tin and indium. At low pressure and relatively high voltages, a discharge can be initiated to develop a high-density plasma region. Injecting an intense beam of electrons into this region causes compression of the plasma and ultimately results in the kinetic energy of the plasma being fully converted into thermal energy and finally electromagnetic radiation having wavelengths in the EUV range. Examples of plasma sources are described in WO 01/99143.
While using wavelengths in the EUV range allows for the fabrication of very small features, it can cause practical problems. Radiation at this wavelength is absorbed in all materials and is therefore not suitable for use with refractive optics. Therefore, the optics in a projection system for use with EUV lithography must be based on mirrors, which can only operate in an ultra high vacuum (UHV) environment. A further problem is that the conversion efficiency, i.e. the ratio of power out at the required wavelength to power in, for discharge sources is very low, which means that the radiation power output is correspondingly low. While the systems described in U.S. Pat. No. 6,570,168 and U.S. Patent Application Publication 2004/0036037 A1 address some of these issues, their physical construction is relatively complex.